Find the first substring in C++

Years ago, I interviewed for a summer internship, and I got a coding question along the lines of: "Implement a function that finds a substring within a string. If that substring is found, return the index of the found substring."

I fumbled through the question. Eventually I got a working solution, but it was a clumsy process getting there. Why was such a simple question so difficult to implement on the spot?

In hindsight, I think this is a good interview question for a few reasons. The problem is deceivingly simple; just about anybody is capable of doing this correctly, but it's difficult to do it well and efficiently, especially on the spot. Because the problem is so simple, the focus of evaluation lies in your style and thought process. Do you clutter your code with unnecessary variables? Is your code verbose or terse? How many functions do you compose, and why? Is your style sloppy or bug-prone? I didn't get that particular internship, because my code was definitely sloppy and bug-prone.

I think there's one more aspect of this question that causes applicants to freeze: strings. In C++ and Java, people tend to use the std::string/String classes, which have lots of utility functions built in. In day-to-day programming, you would rarely iterate over a string manually, and you might also avoid using pointers of type char*.

In this particular example, I think it's actually easier to use pointers than integer indices; this way you have fewer local variables to keep track of. To some extent, having fewer variables means fewer opportunities to introduce a bug.

In my version, I like splitting it up into two functions. The primary function simply iterates over the outer string, and searches each substring for the sought string. Heres' the code:

bool matched(char* sought, char* sent){
    //utility function
    while (*sought!='\0'){
        if (*sent=='\0') //sought is longer than sent
            return false;
        if (*sent!=*sought) //The characters don't match
            return false;
        sent++; sought++; //Check next character
    }
    return true;
}

int substring(char* sought, char* sentence){
    //primary function
    char* iter=sentence; //iterate over string
    while (*iter!='\0'){        
        if (matched(sought, iter)){
                return iter-sentence;
        }        
        iter++;
    }
    return -1;
}

As you can see, the pointers can reduce clutter in the code, because they already are indices in an array of characters.

Reversing C-Style Strings With Pointers

It's a common interview question to ask an applicant to reverse a c-style string, without using string utility functions. In practice, this is something you almost never do, as modern programming languages have nice, safe string classes and interfaces, most likely complete with their own string::reverse() methods.

However, the test has some value. Depending on how the applicant approaches the task, you can gauge his comfort level with pointers. You can get a feel for the applicant's efficiency with memory and operations. Do they reverse the string in place, or allocate new memory?

In particular, swapping the values of two variables, is an interesting sub-test. Can the applicant do this correctly? How do they choose to do it? They can do it in a separate function, with pointers, with references, etc.

Finally, testing. Does the applicant simply write an answer and say "tada"? Or do they try to walk through the code and test what it does, maybe catching mistakes or corner-cases?

As long as the applicant can correctly complete the task, there are no wrong answers. This test really elucidates their style and thought process.

Even though a person might scoff at such a "trivial" test that "doesn't prove anything," it turns out that the test is not so trivial for most programmers. Notably, most applicants cannot correctly complete this task the first time.

Now to the code:

1:  void reverse_str(char* str){  
2:       //Receive a c-style string, meaning a pointer to an array of type char,  
3:       //Reverse in place  
4:       char* lower=str;  
5:       char* upper=lower;            
6:       while (*upper!='\0')     //Searching for the end of a null-terminated string       
7:            upper++;       
8:       upper--;          //Decrement it; we want the last non-null element  
9:       while (upper>lower){       
10:            char temp=*lower;  
11:            *lower=*upper;  
12:            *upper=temp;                                          
13:            upper--;  
14:            lower++;  
15:       }  
16:  }  

In this example, I've used pointers. To find the length of the string, I iterate the pointer [upper] until I find the null-character. I then decrement this pointer, to the last non-null character.

Then, I swap the values of the lower and upper pointers, incrementing the lower and decrementing the upper, until they meet in the middle. And that's it.

If I wanted to be a little more elegant with my swapping, I might write a separate function to swap the values of the pointers. How might I do that?

1:  template<class T>  
2:  void swap(T* a, T* b){  
3:       //Swap the values of two pointers  
4:       T temp=*a;  
5:       *a=*b;  
6:       *b=temp;  
7:  }  

In this case, I've chosen to swap using pointers. So, lines 10-12 in reverse_str() would change to:

1:  swap(lower,upper);  

One other option could be to swap using references. It's actually pretty important that an applicant understands references in C++, as this is a common and integral feature of the language. [Maybe I'll do a follow-up entry on this at some point]. Here's a swap with references:

1:  template<class T>  
2:  void swap(T& a, T& b){  
3:       //Swap the values of two variables  
4:       T temp=a;  
5:       a=b;  
6:       b=temp;  
7:  }  

In this case, lines 10-12 of reverse_str() would need the pointers to be dereferenced:

1:  swap(*lower,*upper);  

So yeah, there you go. There are myriads of other ways to approach this problem. For such a seemingly simple task, you get a lot of mileage and information about a software engineer.

Binary Search With Pointers, C++

Today I'm posting my version of binary search, using pointers in C++. Binary search is a fundamental algorithm, which searches for an element in a sorted array. It is considered the mother of all divide-and-conquer algorithms, and executes in log(N) time, where N is the size of the array. The log is taken with respect to base 2 [binary].

Implementing Binary Search with pointers is a nice exercise that brings together a few concepts.

1. While binary search is an important algorithm from a conceptual standpoint, it's also highly practical and has come up in my research and work. Consider captured data that is indexed by its timestamp, e.g.

                timestamp :  value 1, value 2,....value N
             
The most efficient way to search for the values at a particular timestamp is binary search. In reality, we may want the nearest timestamp to a given reference, but that's a minor adjustment to the algorithm.

2. Implementing binary search with pointers helps to reinforce understanding of pointers, a concept which tends to trip up many C++ programmers.

3. Despite the fact that binary search is a fundamental algorithm, very few job applicants are able to code it correctly on a whiteboard. In short, it's good practice.

In our example, we initialize an array with random elements, then sort it. First we demonstrate how we find an element in the array. Second, we demonstrate how the function behaves when we search for an unknown element. Now, to the code:

 #include <iostream> //For std::cout  
 #include <algorithm> //For std::sort  
 #include <stdlib.h> //For srand, rand   
 #include <time.h>  //For time   
   
 template<class T> //template class means we can accept arguments of different classes: int, float, double, etc.  
 int binarysearch(T* arr /*pointer to beginning of array*/, T* low, T* high, T searchkey /*seeking this value in array*/){  
      int index=-1; //Default value, if we don't find searchkey  
      if (*low>*high) //Double-checking that the array's actually sorted  
           return index;  
      if (low>high) //At this point, we've searched the whole array  
           return index;  
   
      T* mid=(high-low)/2+low; //The array index midway between high and low  
      if (*mid==searchkey)   //We've found the searchkey  
           return mid-arr;  
      if (searchkey<*mid)  
           return binarysearch(arr,low,mid-1,searchkey); //Recursive call to search the lower half of the array  
      else if (searchkey>*mid)  
           return binarysearch(arr,mid+1,high,searchkey); //Recursive call to search the upper half of the array  
   
      return index;  
 }  
   
 int main(int argc, char* argv[]){  
      //Using srand to initialize rand() with time()  
      srand (time(NULL));  
   
      //Size of our array will be 15  
      int size=15;   
      int* arr=new int [size]; //Note that arr is the array, and is also a pointer. It points to a memory block of 15 integers.  
      for (int i=0; i<size; i++){            
           arr[i]=rand()%100; //Randomly initialize the elements of the array, integers between 0 and 100  
           std::cout<<arr[i]<<",";  
      }  
      std::cout<<"\n";                 
      std::sort(arr,arr+size); //Sort the array  
      for (int i=0; i<size; i++)  
           std::cout<<arr[i]<<",";  
      std::cout<<"\n";  
   
      //Finding element in array.   
      int to_find=arr[rand()%size]; //Choosing an element at random from the array to search using binary search  
      int found_index=binarysearch(arr,arr,arr+size-1,to_find);  
      if (found_index>-1)  
           std::cout<<"Found "<<to_find<<" at location "<<found_index<<"\n\n";  
      else  
           std::cout<<"Couldn't find "<<to_find<<" in array\n\n";  
   
      //Finding unknown element  
      to_find=100; //We initialized the array with elements strictly smaller than 100  
      found_index=binarysearch(arr,arr,arr+size-1,to_find);  
      if (found_index>-1)  
           std::cout<<"Found "<<to_find<<" at location "<<found_index<<"\n";  
      else  
           std::cout<<"Couldn't find "<<to_find<<" in array\n";  
   
      delete [] arr; //Clean up dynamically allocated memory  
      return 0;  
 }